This invention relates to a low bit-rate speech coding method and a device therefor. The low bit-rate speech coding method or technique is for coding an original speech signal into an output code sequence of an information transmission rate of less than 16 Kbit/sec. The output code sequence is either for transmission through a transmission channel or for storage in a storing medium. The output code sequence is decoded by a decoder where the original speech signal is reproduced by synthesis. The speech coding method is useful in, among others, mobile radio communication, speech synthesis, and voice mail.
Speech coding based on a multi-pulse excitation method is proposed as a low bit-rate speech coding method in an article contributed by Bishnu S. Atal et al of Bell Laboratories to Proc. ICASSP, 1982, pages 614-617, under the title of "A New Model of LPC Excitation for Producing Natural-sounding Speech at Low Bit Rates". As will later be described more in detail with reference to one of more than ten figures of the accompanying drawing, speech synthesis is carried out according to the Atal et al article by exciting a linear predictive coding (LPC) synthesizer by a sequence or train of excitation or exciting pulses. Locations or positions and amplitudes of the excitation pulses are decided by the so-called analysis-by-synthesis (A-b-S) method It is believed that the method of Atal et al is prosperous as a method of coding speech signals at a bit rate between about 8 and 16 Kbit/sec. The method, however, requires a large amount of calculation in determining the locations and the amplitudes.
An improved "voice coding system" is disclosed in U.S. patent application Ser. No. 565,804 filed Dec. 27, 1983, by Kazunori Ozawa et al, assignors to the present assignee (Canadian Patent Application No. 444,239 filed Dec. 23, 1983). The specification of the Ozawa et al patent application will hereinafter be referred to as an elder or prior patent application. The voice or speech coding system of the elder patent application is for coding a discrete speech signal sequence into an output code sequence, which is for use in exciting a synthesizing filter in a decoder. The discrete speech signal sequence is divisible into segments, such as frames of the discrete speech signal sequence.
As will later be described more in detail, the system of the elder patent application comprises a K parameter calculator responsive to each segment of the discrete speech signal sequence for calculating a parameter sequence representative of a spectral envelope of the segment, an impulse response calculator responsive to the parameter sequence for calculating an impulse response which the synthesizing filter has for the segment, an autocorrelator responsive to the impulse response sequence for calculating an autocorrelation function of the impulse response sequence, a cross-correlator responsive to the segment and the impulse response sequence for calculating a cross-correlation function between the segment and the impulse response sequence, an excitation pulse sequence producing circuit responsive to the autocorrelation and the cross-correlation functions for producing a sequence of excitation pulses by successively deciding locations and amplitudes of the excitation pulses, a first coder for coding the parameter sequence into a parameter code sequence, a second coder for coding the excitation pulse sequence into an excitation pulse code sequence, and a multiplexer for combining the parameter code and the excitation pulse code sequences into the output code sequence.
With the system of the elder patent application, locations of the respective excitation pulses and amplitudes thereof are decided with a drastically reduced amount of calculation. It is to be noted in this connection that the locations and the amplitudes are calculated assuming that the amplitudes are dependent solely on the respective locations. The assumption is, however, not generally applicable to actual original speech signals, from each of which the discrete speech signal sequence is produced.